How Low can Water Flow Meters Go

How Low can Water Flow Meters Go


How Low can Water Flow Meters Go

Water flow meters are great devices to measure the flow of water for domestic and industrial use. But how low can a water flow meter go when measuring the flow of water?

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Advanced technology has made measuring fluids and water easy and accurate. Water needs conservation since it’s a valuable commodity. That’s what makes water flow measuring a big business.


Each year, water is a more valuable commodity, and, each year, measuring water is a more significant business. A water flow meter is used to track smaller water quantities for leakages and theft. It is a means of ensuring accuracy when measuring water. This article will focus on devices used to measure water flow and what happens when measuring water in low flows. We are also going to check on the accuracy of these instruments when they measure low flows.


Today, water flow measuring devices include paddle wheels, compounds, turbines, magnetic and ultrasonic flow meters. These are the most used water flow measuring equipment. Other measuring devices include differential pressure (DP) and positive-displacement (PD) flowmeters. Apart from DP and PD meters, all the other flow meters we’ve mentioned can measure flowing velocity directly.


Depending on the meter’s cross-sectional area, a volumetric measurement is calculated from the measured velocity. We will be looking at water flow meters that measure velocity.

How Low can Water Flow Meters Go

When dealing with a wide range of measurements, accuracy is important in reports given. To get the turndown ratio, the highest flow is divided by the lowest flow possible. Let’s say, for example, the high flow to be measured is 1000 GPM and 10 GPM is the low flow to be measured. The turndown ratio will be better than 100:1. If the low flow is ten, and the high flow to be measured is 100, the turndown ratio will equal to 10:1 or better. When choosing a water flow meter, users need to look for meters that give accurate results in the flow range covered. This is called “usable turndown.” Whether it’s a low or high flow, measuring velocity is important.


The turndown ratio is experienced in all flow meters. The turndown is given as a ratio(10:1), as a range of measurement(1-100GPM), or as a velocity span(0.3 FPS to 20 FPS). What matters is that the range is “usable” for the flow measurement being made.


A flow meter that registers 1200 doesn’t mean that it will always register to get that high flow. Even though a flow meter registers a turndown of 100:1, the velocity at 1200 GPM would be 30.6 feet per second in a nominal 4-inch pipe. This happens at 1000 GPM 25.5 feet per second. For water distribution velocity, such figures are not normal due to high friction losses and pumping costs. This is where velocity comes in. A good example is when a flow meter registers 1200 GPM to 12 GPM and a measured value accuracy of 0.5%. For a 4-inch ultrasonic or magnetic flow meter, this is a typical stated accuracy and range. Compared to a 4-inch turbine water flow meter, which has 1.5% of accuracy measured value. Its occasional flow is 1200 GPM and a continuous flow of 1000 GPM. In a 4-inch pipe, the normal expected highest flow ranges from 300 GPM to 450 GPM. This translates to a 7.5 to 10 feet per second velocity rate. Manufactures of such flow meters state that these meters are capable of a “usable turndown” of 100:1. In a real sense, it ranges from 25:1 to 30:1.


What is the velocity at 12 GPM low flow rate of a foot per second? 


This is the question that engineers try to find out is such a low flow rate and even lower flow rates.


Accurate accuracy


Another fact that is considered in flow meters is their accuracy statement. It is normal for magnetic flow or ultrasonic flow meters to register a 0.5% accuracy statement of the measured value. In the turbine flow meters, the normal measured value is 1.5%.


Such flow meters give accuracy statements of the measured value. This means that within a specified flow range, the flow is lower or higher, and the accuracy will remain the same. For instance, at 100 GPM or +/- 5 gallons at 1000 GPM, a stated accuracy will be 0.5% of +/- ½ gallon.


In other mechanical meters like the paddlewheel water flow meter, its maximum published range determines the stated accuracy. In a 4-inch pipe, the range is a flow rate of 12 GPM to 783 GPM for a paddle-wheel meter. This equals 0.3 to 20 feet per second velocity. The error could go as high as 7.83 gallons (1% of the maximum range) or 65% of the measured value if we go by these parameters at 12 GPM.


The error will vary if accuracy is stated as a function of the maximum measuring range. In lower flow conditions, this is more important.


If the flow is steady at 250 GPM and the flow meter is properly specified, turndown doesn’t matter. When a device works at an upper range, maximum range and measured value accuracy are equal. This means that a more expensive DP meter will sometimes serve the same purpose as an inexpensive paddle-wheel. For optimal performance when using a flow meter, the application should match the device used.




Flow meters are essential components in measuring the flow of water. Even though they are not as expensive as other water system devices, flow meters serve a crucial part in determining water usage. If the flow meter is faulty and gives inaccurate reports, the chances are that the end-user will get inflated water bills at the end of the month. Flow meters can also underreport, translating to greater distribution of water that is not required leading to losses. The best thing to do is choosing a water flow meter that gives accurate reports based on the widest “usable” turndown.

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